The current AASHTO protocol for determination of resilient modulus of soils and aggregate material (T307-99) is based largely on Long Term Pavement Performance (LTPP) Protocol P46. The LTPP protocol evolved over a number of years and has had numerous contributors (including significant guidance and input from the authors of this paper). To-date, over 3000 samples have been tested with P46 and results of this testing effort will have far-reaching implications in the development of performance models for pavement structures. Many lessons were learned during development of P46 and this history is documented in a companion paper. The present paper provides a background of the reasons and rationale behind some of the major technical aspects of P46, and by direct association, AASHTO T307. The paper also offers suggestions for improvement or modification of T307. It is hoped that this discussion will lead to a deeper understanding of the test procedure and perhaps facilitate a discussion of the direction the T307 procedure should follow in the future.
In light of the data collected under the Seasonal Monitoring Program of the Long-Term Pavement Performance Program, the relationship between climatic factors and pavement structural properties is investigated. Using deflection data collected at a seasonal site over the course of 1 year, layer elastic moduli are backcalculated. The correlations between pavement structural properties (represented by layer elastic moduli) and climatic factors (represented by pavement temperature and moisture conditions) are studied using a series of statistical analyses. Bivariate analysis is used to evaluate linear association between pairs of variables. Principal components analysis is used to identify principal constructs of the system. Multiple regression is used to calibrate linear models. Numerical optimization is used to calibrate nonlinear models that correlate backcalculated layer moduli to pavement temperature. The latter models are then used to derive a temperature correction algorithm of backcalculated asphalt concrete elastic modulus.
Time domain reflectometry (TDR) is a new technique that can be used to measure indirectly the in situ volumetric moisture content of soil. A growing body of research has been conducted in providing a variety of prediction equations to estimate the volumetric moisture content using the dielectric constant calculated from the apparent length obtained from the TDR reader. However, limited research has been conducted to determine which of several available procedures should be used to obtain the apparent length of the TDR response to be used in calculating the dielectric constant. As a result, evaluating which procedure yields the most accurate assessment of the volumetric moisture content of soils is the object of this paper. There are five known methods of analyzing the apparent length of TDR responses. They are the method of tangents, method of peaks, method of diverging lines, alternate method of tangents, and the Campbell scientific method. Twenty-eight soil samples, from the FHWA seasonal monitoring program, were obtained throughout the United States and Canada and used in a laboratory study. Three levels of moisture and five levels of compaction were initially planned for use with each soil sample. A total of 361 data points were eventually obtained and used to analyze each method. The method of tangents proved the most accurate method of estimating the volumetric moisture content. Current studies are ongoing to provide improved multiple regression models to estimate the volumetric moisture content on highway soils.
The theoretical fundamentals used in evaluating Poisson's ratio and elastic modulus of materials using the indirect diametral tensile test are evaluated. With the current state of practice (ASTM D4123), the material properties are evaluated by the two-dimensional stress equations for a circular element supporting short-strip loading along the vertical diameter. Because of the inability of these equations to study misalignment of the specimen, the planar solution is analyzed. The analysis of the above two approaches indicates that the material properties predicted are relatively insensitive to specimen size and misalignment. However, the influence of aggregate inclusions in the vertical plane may cause significant propagation of errors in the vertical measurements outside the central half-radius that significantly affects the value of the predicted Poisson's ratio. The influence of aggregate inclusions in the horizontal plane does not appear to be a significant factor contributing in the horizontal displacement variations. Thus, determination of the elastic modulus from horizontal displacements alone has great potential in providing consistent, reasonable results with an assumed Poisson's ratio. In addition, a means of estimating the magnitude of the displacements and the required sensitivity of the measuring devices based upon expected Poisson's ratio and gauge length is presented. Finally, test control parameters based upon the ratio of vertical to horizontal deformations have been developed to check if the material being tested is within the elastic range as the test progresses.
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